1. Field of the Invention
The present invention relates to heat exchange devices and more particularly to evaporative heat exchangers. It also relates to such devices that can also operate to provide heat recovery in combination with ventilation.
2. Description of the Related Art
An evaporative cooler is a device that uses the latent heat of evaporation of a liquid to provide cooling. The principle of evaporative cooling has been known for many centuries. For example, a damp cloth placed over an object will keep the object cool by evaporation of liquid from the cloth. By continuously adding liquid to the cloth, the cooling effect may be maintained indefinitely without input of electrical energy. The lowest temperature that can be reached by evaporation of moisture in this way into an air stream defines the wet-bulb temperature for that air. An indirect evaporative cooler makes use of this principle. A product air stream over a primary surface of a heat exchange element may be cooled by a working air stream passing over and absorbing moisture from a secondary wetted surface of the heat exchanger.
According to theory, if a quantity of air is cooled by direct evaporation its absolute humidity increases due to the uptake of moisture. Its relative humidity also increases due to its lowered temperature until at the wet bulb temperature it is full saturated with water vapour. If the air is cooled without direct evaporation however, its absolute humidity remains the same. As its temperature decreases only the relative humidity increases until full saturation of the air is reached at the so-called dew point. The dew point is thus lower than the wet bulb temperature and is in fact defined as the temperature to which a body of air must be cooled to reach saturation or 100% relative humidity. At this point, water vapour in the air condenses.
Attempts have been made to improve on the principle of indirect evaporative cooling by cooling or drying the working air stream prior to evaporation taking place. A particularly convenient way of cooling the working air stream is to feedback a portion of the cooled product air. Such devices are often referred to as dewpoint coolers as they may lower the temperature of the product air to below its wet bulb temperature and close to the dewpoint. By optimising the surfaces with which the air streams exchange heat, highly effective heat transfer can be achieved. This has been found especially significant in the case of the heat transfer from the wetted secondary surface. In order to provide moisture to the working air stream, the wetted secondary surface may be provided with some form of liquid supply e.g. in the form of a hydrophilic layer. The presence of such a layer can however result in increased thermal isolation of the secondary surface from the working air stream, thus reducing heat transfer.
A particularly efficient form of dewpoint cooler is known from PCT/NL03/00153, the contents of which are hereby incorporated by reference in their entirety. While not wishing to be bound by theory, it is believed that the success of this device is due at least in part to the presence of heat transfer elements on the primary and secondary surfaces. These heat transfer elements may be in the form of fins and are believed to improve transmission of heat from the primary surface to the secondary surface. The fins act both to directly conduct heat and also to break up the various boundary layers that develop in the flow. They also serve to increase the total area available for heat exchange on the relevant surfaces. Further important features of the wetted second surface are known from that document and also from copending UK patent application No 0324348.2, the contents of which are also incorporated by reference in their entirety. Accordingly, by careful choice of the material used as a water retaining layer, optimal evaporation may be achieved without thermal isolation of the secondary surface from the working air stream.
Such devices are extremely convenient for cooling as they are simple to produce and require no refrigerant or compressor. Air may be circulated through the cooler using a low pressure fan which has low energy consumption and is relatively silent. This makes the dewpoint cooler ideal for domestic use, especially at night.
The driving force for cooling in an evaporative cooler is the temperature differential between the wetted heat exchange surface, the working air passing over it and the flow of product air. The greater the efficiency of the cooler and the closer the dewpoint is approached, the more critical is the balance between these temperature differentials. For an evaporative cooler communicating between ambient air and the interior of a building or vehicle, the prevailing wind and pressure differentials can upset this fine balance. Furthermore, the performance of the cooler is dependent upon the inlet and outlet configurations. On installation of an evaporative cooler e.g. in a building using conduits to supply the product and/or working air streams, it may be necessary to carefully calibrate the unit to operate efficiently according to the relative flow resistances of the product and working air conduits.